Summary 11: NEARSHORE BREAKWATERS

Appropriate locations High value frontages with low rates of longshore transport, and weak nearshore tidal currents.
Costs High, but low maintenance (£40,000-£100,000/100m of structure, plus minor works for unprotected)
Effectiveness Cause lee side accretion and erosion behind gaps. Offers good protection within enclosed bays, but potentially damaging to open coasts.
Benefits Dunes not directly disturbed, increases area of dry upper beach, may allow new foredunes to stabilise. Unlimited structure life.
Problems Visually intrusive, alter upper beach morphology, may cause fine sediment, seaweed or debris to accumulate along upper beach. Can cause locally strong currents and may be a hazard to beach users.

General description

Nearshore breakwaters are segmented, shore parallel structures built along the upper beach at approximately high water mark. They are normally built of rock, but can be formed of concrete armour units. At maximum tide levels their crests are still visible, but they may be separated from the shoreline. The gaps allow some wave energy to reach the upper beach and even the dune face.

These structures are distinguished from Artificial Reefs (Summary 10) that are built further down the foreshore and are submerged at high tide.

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Nearshore breakwaters on the upper foreshore in the Dornoch Firth, shortly after construction.

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The same breakwaters as illustrated above, seven years after construction. Note relative stability of backshore.

Function

Breakwaters reduce the energy of waves reaching the shoreline, but do not completely isolate dunes from the natural beach processes. The structures act as a direct barrier to waves, but at very high water levels they allow some overtopping. The gaps between segmented structures allow some wave energy to reach the upper beach and dune face, but this is dissipated by refraction and diffraction. Erosion may continue in the lee of the gaps leading to formation of an embayed shoreline as sand moves into the shelter of the structures.

Sand build up in the lee of the structures (salients) may grow seawards sufficiently to connect with the structure, forming a “tombolo”. If the salient is stable, new foredunes may develop. Recycling or nourishment followed by fencing, thatching and transplanting may be used to accelerate formation of stable salients and dunes.

Breakwaters can have a strong influence on longshore drift and should not normally be used on long expanses of open coast or within estuaries if strong wave or tidally induced currents are present. Breakwaters can cause downdrift erosion or result in dangerous conditions for beach users.

Nearshore breakwaters are distinct from the much larger “detached”, “offshore” or “island” breakwaters that are also not submerged at high tide but are built further down the foreshore or even beyond the low water mark. These alternative forms are generally regarded as inappropriate for most UK coastal situations as they are extremely intrusive on the landscape, expensive to build and are very difficult to design with any degree of confidence regarding their long term effect on the local and adjacent shorelines.

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Cross section of a nearshore breakwater protecting an eroding dune face

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Plan view of a system of nearshore breakwaters

Methods

Breakwater schemes can have a significant impact on the shoreline and should not be implemented without specialist assistance from a competent coastal consultant and contractors. Information on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering”. The accompanying figures provide initial guidance but this should be confirmed for each site.

As with all rock structures on the shoreline the rock size, face slopes, crest elevation and crest width must be designed with care. Rock size is dependent on incident wave height, period and direction, structure slope, acceptance of risk, cross-sectional design, and the availability/cost of armour rock from quarries. In general 3-6 tonne rock will suffice, provided that it is placed as at least a double layer, with a 1:1.5 to 1:2.5 face slope, and there is an acceptance of some risk of failure. Rock size may need to increase if the structures are built further down the beach face where wave action is stronger.

Randomly dumped rock with a high void to solid ratio is hydraulically more efficient than placed and packed rock. However, rock structures on recreational beaches should be built with a view to minimising the potential for accidents involving beach users slipping between rocks.

The structures should be constructed within a shallow trench and a geotextile filter should be laid under the rocks to prevent the migration of sand upwards and the settlement of the rocks into the beach. The geotextile should be wrapped around the base layer of rocks, and the rock toe should be set below the lowest expected beach level.

The ends of each breakwater should be formed into a roundhead with shallower side slopes, particularly along the landward face. The roundhead reduces the tendency for local scour and improves the stability of the structure and the units are normally considered to be more unattractive than rock armour.

Concrete armour units of various types can be used instead of rock, but are normally considerably more expensive. The potentially greater hydraulic efficiency of the units is of no importance to a dune defence structure and the units are normally considered to be more unattractive than rock armour.

There is little guidance available for the length, gap widths, crest heights or distance offshore. Numerical modelling by competent coastal consultants should be undertaken during preliminary studies. As a rough guide gaps should be about half of the structure length and the length should be about equal to the distance from the initial shoreline. As the gaps widen there will be more movement of sand from the bays to the salients. Provided that no backshore assets are at risk this process can continue until tombolos form in the area protected by the breakwaters. Structure and gap lengths can vary along the scheme to provide varying levels of backshore protection. Structures should be constructed on the upper foreshore so that they can be of modest height, yet be effective in protecting the dune toe. The seaward toe should be at or above HWMOST and the crest about the level of maximum wave run-up.

The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both winter storms and more normal wave conditions. The toe of a freshly eroded dune face is normally just below the run-up limit of the most recent severe sea.

The structures should be relatively modest in size, encompassing a small part of the foreshore, so as to have only a small impact on the rate of littoral drift. Breakwaters should not be used in areas where there is a moderate to strong littoral drift since they could cause downdrift erosion by cutting of the supply of material in that direction.

If there is not a large natural supply of sediments it may be necessary to recycle or import beach nourishment material (Summaries 5 and 7) to create salients in the lee of the breakwaters. Once beach material has built up above the highest tide levels, the foot of the dunes should be safe from wave action. If there is a significant build up of sand, fencing and transplanting can be used to encourage dune growth (Summaries 2 to 4).

Costs for nearshore breakwater schemes depend on structure dimensions and spacings. They can be heavily influenced by the availability of suitable rock (or other material), transport and the costs of any recycling or nourishment. Rock structures can be assumed to have an unlimited life with respect to economic assessments.

Impacts

Even though this form of defence is intended to give only partial protection to the shoreline the impacts on shoreline processes, intertidal habitats and landscape will still be high, and may be unacceptable in environmentally sensitive areas. Erosion in the lee of the gaps may well continue for several years after construction while a new beach planshape develops.

On frontages affected by longshore transport the breakwaters may reduce drift rates, resulting in the erosion of downdrift stretches of coast, but helping to stabilise the updrift shore.

As the structures may be separated from the shore at peak water levels they are potentially hazardous to anyone using them as a perch and becoming stranded as the tide rises, particularly if there are also heavy seas.

Where the nearshore waters tend to be silty the breakwaters may encourage lee-side deposition of mud leading to both unwanted odours and unsafe beach areas. Other lee side deposits may include sea weed and jetsam from ships (plastic containers, nets, rope, etc)

Wave induced currents around the ends of breakwaters can be locally strong and a danger to beach users.

Best practice and environmental opportunities

The width of the upper beach along the embayed shoreline may increase, providing improved recreation. New foredunes may develop in the lee of the breakwaters. The structures allow natural beach-dune processes to continue, albeit along a modified shoreline. Existing dune habitats and land forms may be retained and/or enhanced in the areas behind the structures.

All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:

In addition to these general guidelines, the following are of specific importance to nearshore breakwaters: